we use the same embed functionality on our site. i assume the discourseUrl and discouseEmbedUrl are not what you posted above and instead your forum’s relevant urls?
otherwise, the code looks ok. i know the embed functionality is working for up-to-date Discourse beta. we’ve had a ton of embedded posts this past week. i find it odd that it is embedding the first part of the post but the show full post button isn’t working. ours load immediately 
did you check the console for errors?
you could always try disabling the embed_truncate setting just to see if it posts the whole text. that might help narrow down the cause of the issue.